An arachnid differs from a crustacean because :

Options

A. arachnids lack antennae
B. arachnids are mostly aquatic and crustaceans are mostly terrestrial
C. only arachnids shed their exoskeletons
D. only arachnids have two major body segments of cephalothorax and abdomen

Explanation

To determine how an arachnid differs from a crustacean, we can follow these steps:

### Step 1: Identify the Groups
- **Arachnids** and **Crustaceans** are both classes within the phylum **Arthropoda**.

### Step 2: Examine Key Features
- **Crustaceans** typically have two pairs of antennae. This is a defining characteristic of this group.
- **Arachnids**, on the other hand, do not have antennae at all. Instead, they have other sensory structures.

### Step 3: Compare Other Features
- While both groups share some common features as arthropods, such as an exoskeleton and jointed appendages, their differences are significant in terms of sensory structures.
- **Arachnids** also possess specialized respiratory structures like **book lungs** or **book gills**, which are not found in crustaceans.

### Step 4: Conclude the Difference
- The primary difference highlighted in the question is that **arachnids lack antennae**, while **crustaceans have two pairs of antennae**.

### Final Answer
- An arachnid differs from a crustacean because **arachnids lack antennae**.

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